can anyone tell me what the flight director does? Does it only work when the autopilot is on?
The essential task of the flight director is to direct the pilot with visual cues. The pilot follows the visual cues with control inputs to take the aircraft to the destination set in the autopilot. When the auto pilot is not engaged, the pilot uses the flight director to manually fly the aircraft. When the autopilot is engaged, the flight director shows the pilot what the autopilot is doing. Whether the autopilot is engaged or disengaged, the flight director is driven by the same values that drive the autopilot.
The flight director reduces the work load by displaying directions from a horizontal situation indicator (HSI), located on the attitude indicator. The HSI allows the pilot to get course, glide slope, and attitude information from just one instrument. Without a flight director, a pilot is required to include the attitude indicator (AI) and a navigation (nav) instrument in their normal instrument scan. The nav instruments provide course and glide slope information and the AI provides information about changes in turns and pitch.
The flight director computes how much bank and pitch are required to maintain the course set by the pilot. It takes the raw data provided by the HSI, which calculates whether the aircraft is right or left of course, and above or below glide slope. The flight director transposes this data onto the AI to indicate the correct pitch and bank that will keep the aircraft on its programmed course. 😉